Light emitting diodes (LEDs) have been widely used as decorative lighting sources because of their physical properties, such as low power consumption, small size and extended lifetime. The market for decorative LED light strings is large.
Currently, most of the conventional LED light strings use a serial structure such as illustrated in FIG. 1 in which of the LEDs 102, 104, 106, 108, 110, 112 are connected in series as shown in the series wired LED string 100 of FIG. 1. The series wired LED string 100 comprises a circuit having an input 114 and output 116. There are three wires that span the length of the string in the circuit of FIG. 1. This structure is also disclosed in U.S. Pat. Nos. 6,461,019 and 6,830,358 which are specifically incorporated herein by reference for all that they disclose and teach. A problem encountered with the series wired LED string 100 is that if a single LED in the string fails because the LED burns out, becomes unplugged or any other reason that may cause an open circuit, the entire string will fail. In other words, a single failure of a LED in the series wired LED circuit 100 illustrated in FIG. 1 will cause the entire string to fail and not illuminate.
To overcome the disadvantages of the series wired LED structure 100 illustrated in FIG. 1, a parallel-series wired LED string 200 has been used which is disclosed in U.S. Pat. No. 7,045,965, which is specifically incorporated herein by reference for all that it discloses and teaches. As shown in FIG. 2, the circuit has an input 216 and output 218 that includes an upper conductive pair 220 and a lower conductive pair 214. The upper conductive pair includes a wire 202 and a wire 204. The parallel-series wired LED string 200 illustrated in FIG. 2 increases the reliability of the light string 200 in comparison light string 100 of FIG. 1. As shown in FIG. 2, wires 202, 204 are connected in parallel to form two parallel-series strings. Interconnecting wires such as interconnecting wires 210, 212 create individual modules such as the parallel connected modules containing LEDs 206, 208. If one of the LEDs 206, 208 fails, the other LED continues to provide a conductive path in the upper conductive pair 220. For example, if LED 206 fails and creates an open circuit, the conductive path continues through LED 208 in the upper conductive pair 220.
A disadvantage with respect to the circuit illustrated in FIG. 2 is that there are four wires that span the length of the string in the circuit of FIG. 2. Since the cost of the wires is the dominant cost for LED light strings, the competitiveness of the parallel-series wired LED string 200 of FIG. 2 is diminished. Also, if both LEDs in a module burn out or otherwise create an open circuit, the entire string will not illuminate.